The present invention relates to refrigeration control devices. More particularly, the present invention relates to a device for overriding the pressure control starting switch for a compressor on a conventional refrigeration system having a pump-down cycle when the compressor is subjected to low ambient temperatures.
Generally, medium temperature refrigeration units have been one of two types. A first type, and generally considered outdated now, utilizes a compressor and condenser located generally either inside a building, or in the basement of a building. Thus, the compressor and condenser unit in this first type of system is never subjected to near 0.degree. F. temperatures which can cause the refrigerant, normally Freon 12, to change to a liquid state when the system is shut off.
A second type of system, and that normally in use today, utilizes, a compressor and condenser unit that is placed outside the building, and normally either mounted on the roof of the building, or at the side of the building. Therefore, the compressor and condenser unit on a new type of system is periodically exposed to ambient temperatures well below 32.degree. F., and in some portions of the country, below 0.degree. F. Because of this exposure to low ambient temperatures, the new type systems are configured to include a pump-down cycle to prevent the compressor pump from being damaged.
A pump-down cycle is accomplished by installing a solenoid valve in the liquid line of the refrigeration system. When the temperature inside the refrigerated area is satisfied, the thermostat in the refrigerated area interrupts the power to the solenoid valve, and the solenoid valve closes off the liquid line. The compressor continues to run until all of the refrigerant is captured inside a receiver tank that is generally located with the compressor on the condensing unit. When the liquid has been captured in the receiving unit, the compressor is then shut off. The shut-off of the compressor is controlled by a pressure control device that may be set to shut off the compressor at a preselected pressure. The pressure control is normally set to shut off the compressor when the pressure in the suction line of the system reaches 0 pounds per square inch gauge (psig). When the temperature inside the refrigerated space rises to a preselected level, the thermostat in the space returns the power to the solenoid valve which opens the liquid line. Normally, the refrigerant pressure rises in the system to a pre-set level on the pressure control, and the compressor begins to operate and cool the space again.
The pump-down cycle is necessary because in cold ambient conditions, in a system without a pump-down cycle, the refrigerant will migrate to the coldest point in the system when the compressor is shut off. Where the compressor and condenser are located outside, the coldest point will normally be these units. If the ambient temperature is low enough, this migrated refrigerant will quickly liquify. Upon starting the compressor pump again, the pump will be forced to pump this liquid refrigerant. This action quickly damages compressor pumps because such pumps are not designed to pump any liquids at all, only to pump and compress gases. The pump-down cycle prevents damage to the compressor pump by forcing all of the refrigerant in the system to the receiver tank, which is separated from the compressor pump. Thus, in a system with a pump-down cycle, when the compressor pump is turned on again, the pump will not be forced to pump the liquid refrigerant.
One problem with the new type of systems equipped with a pump-down cycle is that in very cold ambient conditions, the pressure of the refrigerant may not be able to rise to the necessary pressure required to activate the pressure control after the solenoid valve is opened. This condition will prevent the compressor pump from starting, and therefore will prevent the refrigeration system from continued cooling of the refrigerated space, because in a pump-down system, the pressure control is the only control that starts and stops the compressor. Normally, when this condition occurs, a serviceman must be called to start the compressor.
The serviceman can generally start the compressor by one of two methods. First, the serviceman can set the pressure control to turn on at a very low pressure, for example at 0 psig. This method has the disadvantage of causing the compressor to run much longer than necessary, and particularly causes the system to operate in a vacuum much of the time. Operating the system in a vacuum is highly undesirable because contaminants can be drawn into the system which can cause system breakdown and system replacement. The second method that may be used by the serviceman is to install a jumper wire across the pressure control. This method has a disadvantage of causing the compressor to run continuously. This method also has the disadvantage of causing the compressor to run much longer than normal, and causing the system to operate in the vacuum much of the time.
Because of these problems with the new type of systems in low ambient temperature conditions, it would be advantageous to have a device that could bypass the pressure control and start the compressor motor under these conditions. One type of bypass device for bypassing the pressure control of a refrigeration system is disclosed in U.S. Pat. No. 2,191,965 to McGrath. However, the device disclosed in McGrath is usable only on the first type, or older type, of refrigeration systems without a pump-down cycle. McGrath discloses a system in which either the thermostat 20 or the low pressure control 21 can regulate the temperature of the space to be cooled. The respective settings on these two controls determine which of the controls will regulate the temperature inside the refrigerated space. McGrath discloses that the use of two controls to start the compressor is to insure that the evaporator is adequately defrosted during each off cycle of the system.
In the McGrath device, when the refrigerated space temperature rises to a preselected level, the compressor does not immediately turn on. Instead, the low pressure control 21 causes the starting of the compressor to be delayed until the refrigerant pressure reaches 30 psig. Forcing the compressor to delay starting until the refrigerant pressure reaches 30 psig insures that the evaporator is adequately defrosted before the compressor starts. Because the old type of refrigeration systems sometimes located the compressor and condenser units in the basement of the buildings, it was possible for the basement temperature to sometimes fall below about 30.degree. F. If the temperature fell below 30.degree. F., the refrigerant pressure sometimes would not reach the required 30 psig to cause the low pressure control 21 to start the compressor. To insure that the compressor would start under these conditions, McGrath discloses a low ambient temperature control 22 that bridges across the low pressure control 21 whenever the ambient temperature is below 30.degree. F. In McGrath's device, the low pressure control 21 is continuously bridged as long as the temperature remains below 30.degree. F. Thus, the thermostat 20 becomes the sole control for starting and stopping the compressor, and therefore the defrost period for the system is bypassed.
It is apparent from the above discussion that the device disclosed in McGrath could not possibly be used on a new type refrigeration system having a pump-down cycle. If the McGrath device were so installed, whenever the ambient temperature was below 30.degree. F., the compressor would be forced to run continuously because on a pump-down system, the low pressure control is the only control that starts or stops the compressor. Because it is undesirable to run the compressor continuously, the McGrath device would not solve the problems related to low ambient conditions on a new type refrigeration system.
It is therefore one object of the present invention to provide a pressure control override apparatus that is usable on a refrigeration system having a pump-down cycle, and in which the pressure control is the only control that starts and stops the compressor.
It is another object of the present invention to provide a pressure control override apparatus that is activated only when both the ambient temperature around the compressor and condenser units is below a preselected level, and when the refrigerated space temperature rises to a preselected level indicating that cooling within the space is required.